This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Multifrequency electron spin resonance (ESR) spectra provides a wealth of structural and dynamics information about the local environment of the spin label, and indirectly about the proteins to which they are attached. Unambiguously relating the features of the observed spectra to the underlying molecular motions and interactions is, however, challenging. To progress toward a rigorous interpretation of ESR spectra, we performed extensive molecular dynamics (MD) simulations of fully solvated T4 Lysozyme, labeled with spin labeled at positions 72 and 131. These two sites have been the object of numerous experimental studies, and have established themselves as prototypical solvent-exposed sites on the surface of alpha-helices. To extend the time window afforded by the simulations, stochastic Markov models, reflecting the dynamics of spin label side chains in terms of their rotameric states, are constructed from the trajectories. Without adjustable parameters, the multifrequency ESR spectra calculated at three different magnetic field strengths for positions 72 and 131 were found to be in quantitative agreement with experiment. During dynamics the spin label has access to a fairly large number of conformations and displays a significant propensity to form interactions with protein residues other than the nearest neighbors along the helix. The detailed picture of the spin label emerging from the MD simulations unifies the diverse spectroscopic and crystallographic data and provides unprecedented insight into their molecular origins.